THERMOPHOTOVOLTAIC CONVERTERS WITH SOLAR POWERED HIGH TEMPERATURE EMITTERS

Transcription

1 THERMOPHOTOVOLTAIC CONVERTERS WITH SOLAR POWERED HIGH TEMPERATURE EMITTERS V.M.Andreev, V.P.Khvostikov, V.D.Rumyntsev, O.A.Khvostikov, P.Y.Gzryn, A.S.Vlsov, N.A.Sdchikov, S.V.Sorokin, Y.M.Zdirnov, M.Z.Shvrts Ioffe Physico-Technicl Institute, 26 Polytechnichesky, , St.Petersurg, Russi emil: ABSTRACT: Results of solr thermophotovoltic (STPV) system study re reported. Suntrckers, sunlight concentrtors nd STPV modules were designed, fricted nd tested t indoor nd outdoor conditions. Developed sunlight concentrtors of refrctive- nd reflective-types with secondry qurtz meniscus lenses ensured the high concentrtion rtio exceeding 7000x, which is necessry for chieving the high efficiency of the concentrtor-emitter system owing to trp escping rdition. Severl types of STPV modules hve een developed nd tested under concentrted sunlight. Temperture s high s 2000 K ws otined in 12 mm di nd 15 mm length tungsten/tntlum emitters in vcuum, illuminted y concentrted sunlight. Photocurrent density of 4.5 A/cm 2 ws registered in photoreceiver sed on 1 x 1 cm 2 GS cells under solr powered tungsten emitter. Cell efficiency s high s 19% hs een mesured in this STPV system with solr powered tungsten emitter heted to ~ 2000 K. ETA = 27% ws estimted for the tungsten emitter spectrum cut-off t λ > 1820 nm. Anlysis of vrious prmeters, influencing the performnce of the developed STPV systems is presented. The wys for the STPV system efficiency increse up to 30% nd higher re considered: - improvement of the emitter rdition selectivity nd ppliction of selective filters for etter mtching the spectr of emitter rdition nd cell photoresponse; - ppliction of the cells with ck-surfce reflector for recycling the su-ndgp photons; - development of the low-ndgp tndem TPV cells for etter utilistion of the rdition spectr. Keywords: Thermophotovoltics, Concentrtors, III-V Semiconductors 1 INTRODUTION In the solr thermophotovoltic (STPV) system, solr rdition is sored nd reemitted s therml rdition efore illumintion of PV cells. Conventionl solr PV systems re strongly determined y the sunlight spectrum nd y the fct tht there is no ck connection etween PV cell nd the Sun. In STPV systems, the optimiztion my imply choice of the emitter spectrum nd possiility to return the unused prt of rdition from the receiver ck to the emitter surfce supplying it y n "dditionl" power. STPV system llows to utilize selective filters/mirrors nd su-ndgp photon reflection to the emitter, which ensures efficiency increse. The more so, s photons emitted y the TPV cells due to rditive recomintion re utilized, the emitter sors lso these photons using their energy. Owing to this effect, TPV cells would operte in the conditions, where the generted voltge is higher thn in the cse of solr PV, when there is no trpping the emitted photons due to rditive recomintion. In solrpowered TPV systems, high-temperture (~ 2000K) emitter in vcuum ul cn e used with good enough "qulity" of rdition. Like in concentrtor photovoltics, the thermophotovoltic conversion of concentrted sunlight is promising for the decrese of the solr electricity cost in comprison with nonconcentrted photovoltics owing to reduction of the PV cell re proportionlly to n increse of the output electricl power density from the PV cells in highconcentrtor STPV systems. The hyrid system with PV conversion (or lighting) for visile prt of sunlight nd with TPV conversion for the infrred prt of solr spectrum cn e lso creted ensuring the increse of BOS efficiency. The possile hyrid solr/fuel thermophotovoltic unit hve the dditionl dvntge: fuel-fired prt of the hyrid system would permit opertion during the night. There re the following key prolems rising t optimiztion of STPV systems: providing the high sunlight concentrtion; tiloring the emission spectrum of the photon emitter; filtering the rdition to utilize photon recycling process nd to reduce the therml impct on the photocells; tndem cell design llowing to increse PV conversion efficiency of rdition from the emitter. These prolems my e interconnected. For instnce, selective filter my e deposited directly on the photocell surfce reflecting long-wvelength rdition ck to the emitter. The role of such filter my ply photocell itself, if there is mirror on its ck surfce, which reflects the su-ndgp photons, nonsored in the PV cell mteril. Theoreticl [1-7] nd experimentl studies [8-14] show n opportunity to chieve high efficiency in STPV systems. For idel system elements, mximl theoreticl efficiency were found to e out 85%, tht is identicl to the efficiency of unlimited stck of tndem cells. Expected in prctice efficiencies of STPV converters re 30-35%. 2 EVALUATION OF STPV SYSTEM EFFICIENCY One of the possile solr TPV systems shown in Fig. 1 consists of sunlight concentrtor (Fresnel lens nd secondry lens), emitter nd photoreceiver of cylindricl shpes. In generl, STPV efficiency depends on the PV conversion efficiency nd the emitter efficiency. Emitter efficiency cn e defined s rtio of the prt of rdition, which cn e utilized for TPV conversion, to the totl energy irrdited y the emitter. It increses with the decrese of the incident light perture of the sorer nd, thus, emitter efficiency increses with incresing the sunlight concentrtion rtio. The following fctors hve een tken into ccount t estimtion of the rel STPV system efficiency. Direct sunlight (850 W/m 2 ) is collected y the concentrtor

2 Air or wter cooling photoreciever hetsink Emitter Sunlight Secondry qurtz lens Qurtz window TPV cells Figure 1: Concept of solr TPV systems with Fresnel lens s primry concentrtor. system with overll efficiency of 90%. Emitter is lck-ody. Prt of emitter rdition, determined y emitter efficiency flls on PV cells. The emitter rdition is scttered y elements of the inner construction (these losses re ccepted equl to 10%) flls on photocells, which occupy 90% of the whole irrdited surfce (view fctor is 90%). Rdition with photon energies hν < Eg returns from PV cells to the emitter with the return efficiency RE = 90%. The rdition with hν > Eg is sored in the semiconductor nd genertes electronhole pirs. Recomintion losses hve the lowest vlue limited y rditive recomintion. No ohmic losses in the cells re ssumed. The cell operting temperture is equl to 50ºC. As is seen from Fig. 2, the efficiency mximum of 33% tkes plce t emitter tempertures of out 2000K in the cells mde of mterils with ndgps in the rnge of ev. This rnge is higher in comprison with the optiml ndgp rnge of ev in the TPV systems with lower efficiency of su-ndgp photon recircultion (RE < 50%) nd t lower emitter temperture[15-17]. It mens tht GS nd InGAs/InP widly used for friction of TPV cells [15-23] re the optiml mterils for STPV systems with high return efficiency nd high emitter temperture (~ 2000 K). STPV module efficiency, % ev 0.9 ev 1.1 ev 0.5 ev RE=90% CR=16000x Blck-ody emitter temperture, K Figure 2: Clculted STPV converter efficiency s function of emitter temperture for the different ndgps of TPV cell mterils from 0.5 ev to 1.1 ev. As is seen from Fig. 3, efficiency of STPV converter sed on GS (Eg = 0.72 ev) increses with concentrtion rtio increse. The drop in the efficiency with lowering down temperture is explined y the decrese of the conversion efficiency of the GS PV cell, owing to the worse mtching the PV cell photoresponse spectrum to rdition spectr t lower emitter temperture. The drop in efficiency t high tempertures is ssocited minly with the rise of losses of rdition, which leves the sorer through the sorer perture (i.e. owing to decrese in emitter efficiency). For these resons, with incresing sunlight concentrtion rtio, n efficiency increse from 18% t 1000x to 33% t 16000x tkes plce, when the emitter temperture of efficiency mximum increses from 1400 K t 1000x to 2000 K t 16000x. This efficiency evlution shows the necessity to design STPV systems with high sunlight concentrtion rtio nd high emitter tempertures of K. GS sed STPV modulle efficiency, % x 2000x 4000x 8000x 16000x RE=0.9 Eg=0.72 ev Blck-ody emitter temperture, K Figure 3: Clculted GS sed STPV converter efficiency versus lck-ody emitter temperture t vrious solr concentrtion rtios from 1000x to 16000x. 3 SUN TRACKER DESIGN FOR STPV SYSTEMS High ccurcy of trcking to the sun is specific feture of the high-concentrtion PV nd STPV methods. Another very importnt requirement for these systems is low cost of suntrckers. The trcker prototypes hve een designed nd uilt for designted cpcities up to 5 kwp [24]. They hd een instlled t the Ioffe Institute (St.Petersurg), t Frunhofer ISE (Freiurg) nd t NREL (Golden). Three rows of STPV modules 2 Additionl sun sensor (21 modules,50*50 cm ech) Min sun sensor Fresnel lens STPV module Suspended frme Ger segment Two gered motors, ttery nd electronic circuit Bse frme Figure 4: Schemtic of the 1kWp solr trcker equiped with STPV modules.

3 Recently n improved 1 kwp trcker ws designed nd uilt in the Ioffe Institute. Trcker consists of two min moving prts (see Fig. 4,5): se pltform moving round the virtul verticl xis y mens of three wheels, nd suspended one moving round horizontl xis. The suspended pltform is frme where STPV modules should e instlled s three steps of stir. Position of the suspended frme cn vry in the rnge of ±45 symmetriclly out horizontl plne. The se frme is driven y one of three wheels moving in lrge rdius with respect to circle (see photogrph in Fig. 5). At norml trcking the motor is switched on periodiclly, fter ech 5-8 seconds. Similr opertion tkes plce for the verticl chnnel. For verticl driving there re two cogwheels nd two ger segments, situted symmetriclly on two sides of the suspended frme. Such design llows reducing free movements of the frmes under wind conditions nd simplifying the trcker structure. For opertion during cold periods with lnket of snow, the trcker is supplied with specil cps on the wheels. Also, these cps will protect ruer covers of the wheels ginst direct sunrys. 4 DESIGN AND PERFORMANCES OF SUNLIGHT CONCENTRATORS FOR STPV SYSTEMS Two-stge sunlight concentrtors with pseudoprolic mirrors nd Fresnel lens were fricted s first stge of the concentrtor systems. Qurtz meniscus lenses with dimeter of 7 cm were used s secondries to increse the concentrtion rtio in times. The secondry lenses hve ntireflection cotings for the spectrl rnge of nm. The sunlight concentrtor shown in Fig. 6 is comintion of four-segment pseudo-prolic compound mirror nd secondry meniscus lens instlled close to the entrnce window of the STPV module. This two-stge concentrtor ensures the sunlight concentrtion rtio of out 6000x, i.e. 90% of input sunlight comes to the 10 mm perture emitter. Figure 6: Four-segment pseudo-prolic mirror (0.45 m 2 in re, 0.75 m focl length) mde of four rss sphericl dish mirrors (with luminium reflection lyer protected y HfO 2 lyer) instlled on the sun trcker. Figure 5: Sun trcker with STPV (on the left) nd PV concentrtor modules (on the right side) sed on Fresnel lens concentrtors instlled t the Ioffe Institute. The trcker is equipped with min (ccurte) sensor nd dditionl one, oth equipped with tndem III-V solr cells nd operting s prt of close-loop system. Min sensor cn lign the trcker with the sun within 0.05 degree of rc ccurcy with cceptnce ngles of ±70. Additionl sensor mkes wider the Est/West turning ngle (up to 270 ). Both sensors re mounted on the suspended frme together with STPV modules. An dvntge of the developed sensors is the protection from the illumintion level chnges cused y light reflections from vrious ojects nd y presence of clouds. For this, the min sensor includes two su-chnnels in oth horizontl nd verticl chnnels. Differentil signls re generted in correspondence with mislignments of the trcker to the sun in zimuth nd elevtion. Simplicity nd consumption of current s low s 45 micromperes from rechrgele 12V-ttery in stnd-y regime re the positive fetures of the developed electronic ord. Overll power consumption for trcking including West- Est returns ws estimted to e no more thn 0.2% of instlled power. Figure 7: Fcet-type pseudo-prolic mirror (0.2 m 2 ) mde of glss (with Al-reflection lyer protected y SiO 2 s first stge of concentrtor system, instlled on the sun trcker. The sunlight concentrtor shown in Fig. 7 consists of 6-segment fcet-type pseudo-prolic mirror (perture re of 0.2 m 2 nd focl length of 0.75 m) nd secondry qurtz meniscus lens. Owing to high qulity of the fricted fcet mirrors the concentrtion rtio of more thn 7000x is ensured y this two-stge concentrtor.

4 The refrctive-type sunlight concentrtor shown in Fig. 8 consists of Fresnel lens (0.36 m 2 in re nd 0.75 m focl length) nd secondry qurtz meniscus lens. 90% of concentrted sunlight is collected in the spot with dimeter of 10 mm. Concentrtion rtio of 4000x is ensured y this concentrtor, the min dvntge of which is its low cost. However, the mteril (PMMA) of the Fresnel lens is chrcterized y poor outdoor stility. Recently, new technology for Fresnel lenses of composite structure ws developed t the Ioffe Institute [25]: the microprisms re formed from the trnsprent silicone ruer contcting with the front silicte glss sheet s protective superstrte. The developed formtion process llows fricting the lens of totl re of 0.6 m x 0.6 m. Such type of Fresnel lenses ensures much etter environmentl stility owing to the use of high stle silicte glss, protecting the Fresnel lens mde of silicone ruer, which is lso chrcterized y high stility under the ction of outdoor conditions. These of Fresnel lenses re very promising for friction of concentrtor PV modules [25] nd hve perspectives for use in low cost STPV systems. from the oject tested to the control lock. The dischrge illumintion spot of the lmp is locted in the first focus of the mirror. In the second focus (t the distnce of 1 m) there ppers n incresed imge of the dischrge spot cross section. The simultor is chrcterized y the following performnce: input electricl power of the lmp is up to 10 kw t continuous mode opertion; lmp efficiency is 55%; opticl system efficiency is 35%; the opticl power up to 2 kw is collected in the focl spot. Distriution of the rdition over the spectrum rnges is s follows: 0.2 kw in the UV region, 0.7 kw in the visile region nd 1.1 kw in the IR region. The use of the secondry meniscus qurts lens llows chieving the opticl power density up to 1 kw/cm 2, tht is equivlent to > suns. At loss of prt of the energy for therml conductivity of tested specimen plced in the focus of the descried simultor, the temperture exceeding 1700 C ws chieved in the tungsten/tntlum emitters plced in vcuum m Figure 8: Two-stge refrctive concentrtor sed on Fresnel lens (primry) nd qurtz meniscus lens (secondry). 5 HIGH-POWER SIMULATOR OF CONCENTRATED SUNLIGHT FOR INDOOR TESTING OF SOLAR TPV CONVERTERS Tking into ccount tht outdoor testing of STPV modules is possile in St.Petersurg during the summer months only, it is very importnt to hve the possiility of indoor chrcteriztion of the developed devices. Simultor of concentrted sunlight (SCS) hs een developed for indoor testing of TPV converters under high intensive (more thn 500 W/cm 2 ) illumintion of the emitter. SCS (Fig, 9,) consists of power supply lock (1), remote control lock (2) nd rditing element (3). The illumintor consists of n ellipticl dish mirror with dimeter of 0.6 m, lmp ignition device (5) nd xenon lmp (4). The opticl scheme of the furnce is shown in Fig. 9,. The lmp is instlled verticlly. The control lock is intended for the ignition pulse voltge supply (30 kv, 0.5 MHz) nd for smooth regultion of the lmp power. The simultor design provides possiility to sustin utomticlly designted lmp mode y fixing feedck nd y delivery of signl Figure 9: Simultor of high intensive solr illumintion with xenon lmp (), opticl scheme of illumintor (): 1 power supply lock; 2 remote control lock; 3 dish ellipticl mirror; 4 ckside reflector; 5 xenon lmp; 6 secondry qurtz lens; 7 STPV module on the tle providing the motion of the module in three dimensions. 6 SOLAR TPV MODULE DESIGNS AND EXPERIMANTAL RESULTS The TPV prts of the modules (Fig. 10) include cylinder-shped tungsten emitters, emedded in vcuum chmer or in rgon. For severl experiments, tntlum emitters were used insted of tungsten ones, ecuse of esier tntlum tretment. Vcuum chmers with qurtz windows for sunlight entrnce were used to prevent the emitters from the oxidtion. STPV modules of conicl nd cylindricl shpes hve een developed. In the conicl system (Fig.10, nd Fig. 11), therml rdition is reflected to PV cells y n Au coted coneshped mirror, nd the PV cells re mounted on flt sement. In the cylindricl system (Fig.10, nd Fig. 12,13), PV cells surround the emitter, eing mounted on the inner side of cooled cylindricl se (Fig.13). Both modules were tested outdoors under direct sun

5 illumintion. Averge direct sun irrdition ws mesured to e 850 W/m 2. Emitter temperture ws registered y oth pyrometer nd W-Re thermocouple. Tempertures in the rnge of K were otined, depending on emitter size nd mteril. temperture rnge of ºC. Contct composition Mo/Ni/Au to the BeO sustrte llowed to solve the prolem of dhesion of GS cells to cermics. GS cell Qurtz window Emitter TPV cells Emitter 1 cm Wter cooling Vcuum Wter cooling Vcuum Wter cooling Figure 10: Schemtic drwings of the developed conicl () nd cylindricl () STPV modules. 1 cm 1 cm Figure 11: STPV module with flt PV receiver () nd tungsten emitter surrounded y conicl reflector (). Plces for mounting the cooling wings Figure 13: PV receivers of cylindricl shpe with forced ir-cooled GS cells (ir-cooling wings re not shown); conicl TPV module with wter-cooled photoreceiver. V oc (V) nd FF 1,0 0,9 0,8 0,7 0,6 0,5 0,4 0,3 0,2 0,1 FF V oc , Tungsten emitter temperture, K η η GS cell efficiency, % Figure 14: Open circuit voltge (V OC, curve 2), fill fctor (FF, curve 3) nd efficiency (curves 1,4) of GS TPV cell s function of tungsten emitter temperture. Efficiency ws estimted under the following rdition conditions: under the full rdition spectr (curve 1) nd under spectr cut-off t λ > 1820 nm (curve 4). 1 cm Figure 12: Solr powered tntlum (12 mm di) emitter t 1800 K inside the ul (filled up with rgon) in the focl spot of Fresnel lens with secondry meniscus lens. GS TPV cells were fricted with the use of the Zn-diffusion nd LPE technologies. In the first experiments the cells were mounted in prllel on hetsink mde of copper (Fig. 13,). To ensure the series connection of the cells, the hetsink sustrte with high therml conductivity must e n electricl insultor [18,20]. For friction of photoreceivers, we hve selected BeO cermics, which hs electricl resistivity of more thn Ωcm with the est therml conductivity of 250 W/mK. The therml expnsion coefficient of BeO cermics is /K eing close to tht of GS in the The photocurrent density J SC = 4.5 A/cm 2, open circuit voltge V OC = 0.49 V nd fill fctor FF = 0.68 hve een mesured in GS cell in the conicl module under the solr powered emitter heted to the temperture of out 2000 K. The cell efficiency of 19% under illumintion y tungsten emitter heted up to K hd een derived from experimentlly mesured PV prmeters (Fig. 14). Cell efficiency s high s 27% ws estimted for the spectrum cut-off t λ > 1820 nm (conditions of 100% return efficiency for su-ndgp photons). 7 CONCLUSIONS The following pproches cn ensure the efficiency increse in STPV systems. One of the min common fetures of thermophotovoltics nd solr photovoltics is tht in oth systems the energy source is chrcterized y

6 wide spectrum. It mens tht the most effective pproch to improvement of the solr system efficiency, which is the multijunction pproch, my e pplied to improvement of the TPV system efficiency. Theoreticl efficiencies of dul-junction TPV cells sed on nrrow ndgp semiconductors exceed 45% in STPV systems with high-temperture emitters nd t high enough return efficiency of 90%. Su-ndgp photons cn e returned to the emitter y inserting rodnd metllic mirror on the PV cell ckside. Mximum reflectnce of 80% ws mesured in GS cells with such ckside mirror. Another wy to return unused energy to the emitter is to insert selective filter etween the emitter nd the cells. The overll efficiency of rel STPV system cn e lso incresed y the use of the selective emitters. Tungsten is slightly selective mteril with emissivity incresing in the visile spectrum rnge. However, more selective mterils re preferle [26-28]. For instnce, 2D- or 3D-texturized tungsten my led to the increse in emissivity in desired wvelength region (sy, µm). All these pproches together should ensure the STPV system efficiency increse up to 30% nd higher. ACKNOWLEDGEMENTS Authors would like to thnk Zh.Alferov, A.Luque, C.Algor, C.Jussud, A.Bett, A.Gomert, W.Durish, W.Toler for support nd fruitful discussions. Authors thnks V.Grilikhes nd O.Chost for the contriution to this work. This work hs een supported y the Europen Commission through the funding of the project FULLSPECTRUM (Contrct SES6-CT ). REFERENCES [1] W. Spirkl nd H. Ries, J.Appl. Phys., 57 (1985) [2] P. A. Dvies, A. Luque, Solr. Energy Mterils & Solr Cells, 33 (1994) 11. [3] A. Luque, A. Mrti, Solr Energy Mteril & Solr Cells, 58 (1999) 147. [4] V. Bdescu, J.Appl. Phys., 90 (2001) [5] N-P. Hrder, P. Würfel, Semicond. Sci. Technol., 18 (2003) S151. [6] G.D. Cody, Proceedings 4 th NREL Conference on TPV Genertion of Electricity (1998), AIP Conf. Proc. 460 (1999) 58. [7] V.M.Andreev, V.P.Khvostikov, O.A.Khvostikov, V.D.Rumyntsev, P.Y.Gzrjn, A.S.Vlsov, Proceedings 6 th Conference on TPV Genertion of Electricity, AIP Conf. Proc. 738 (2004) 96. [8] K. W. Stone, N. S. Ftemi, L. Grverick, Proceedings 25 th IEEE Photovoltic Solr Energy Conference (1996) [9] H. Yugmi, H. Si, K. Nkmuro, N. Nkgm, H. Ohtsuko, Proceedings 28 th IEEE Photovoltic Solr Energy Conference (2000) [10] V.M.Andreev, V.P.Khvostikov, Proceedings of 3 rd JRC Workshop The pth to ultr-high efficient photovoltics (2003) 83. [11] V.D.Rumyntsev, V.P.Khvostikov, P.Y.Gzryn, O.A.Khvostikov, N.A.Sdchikov, A.S.Vlsov, E.A.Ionov, V.M.Andreev, Proceedings of the 6 th Conference on Thermophotovoltic Genertion of Electricity, AIP Conf. Proc. 738 (2004) 79. [12] V.P.Khvostikov, V.D.Rumyntsev, P.Y.Gzryn, O.A.Khvostikov, N.A.Kluzhniy, V.M.Andreev Proceedings of the 19 th Europen Photovoltic Solr Energy Conference (2004) 105. [13] V.P.Khvostikov, V.D.Rumyntsev, M.Z.Shvrts, O.A.Khvostikov, P.Y.Gzryn, S.V.Sorokin, N.A.Kluzhniy, V.M.Andreev, Proceedings of the 6 th Conference on Thermophotovoltic Genertion of Electricity (2004) 436. [14] F.html [15] J.L.Gry, A.El-Husseini, Proceedings 2 nd NREL Conference on TPV Genertion of Electricity (1995), AIP Conf. Proc. 358 (1996) 3. [16] T.J.Coutts, M.W.Wnlss, J.S.Wrd, S.Johnson, Proceedings of 25 th IEEE Photovoltic Solr Energy Conference (1996) 25. [17] T.J.Coutts, Renewle nd Sustinle Energy Reviews, 3 (1999) 77. [18] L.M.Frs, R.Bllntyne, S.-Z.Ye, S.Hui, S.Gregory, J.Keyes, J.Avery, D.Lmson, B.Dniels, Proceedings 4 th NREL Conference on TPV Genertion of Electricity (1998), AIP Conf. Proc. 460 (1999) 480. [19] V.M.Andreev, V.P.Khvostikov, V.R.Lrionov, V.D.Rumyntsev, M.Z.Shvrts, S.V.Sorokin, V.I.Vsil ev, A.S.Vlsov, Proceedings of the 2-nd World Conference on Photovoltic Energy Conversion (1998) 330. [20] T.Schlegl, F.Dimroth, A.Ohm, A.W.Bett, Proceedings of 6 th Conference on Thermophotovoltic Genertion of Electricity, AIP Conf. Proc. 738 (2004) 285. [21] C.Algor, D.Mrtin, Proceedings 5 th Conference on TPV Genertion of Electricity (2002), AIP Conf. Proc. 653 (2003) 452. [22] V.M. Andreev Solr cells for TPV converters, Chpter 11 in Next Genertion Photovoltics. High Efficiency through Full Spectrum Utiliztion Ed. A.Mrti, A.Luque, IoP (2004) 246. [23] L.B.Krlin, B.Y.Ber, P.A.Blgnov, M.M.Kulgin, A.S.Vlsov, Proceedings of the 5 th Conference on TPV Genertion of Electricity (2002), AIP Conf. Proc. 653 (2003) 373. [24] V.D.Rumyntsev, V.M.Andreev, N.A.Sdchikov, A.W.Bett, F.Dimroth, G.Lnge, Proceedings of the Conference PV in Europe, (2002), 521. [25] V.D.Rumyntsev, A.E.Chlov, E.A.Ionov, V.R.Lrionov, V.M.Andreev Proceedings of the 19 th Europen Photovoltic Solr Energy Conference (2004) [26] A.Gomert, Proceedings 5 th Conference on TPV Genertion of Electricity (2002), AIP Conf. Proc. 653 (2003) 123. [27] S.Y. Lin, J. Moreno, J.G. Fleming, Appl. Phys. Lett., 83 (2003) 380. [28] B. Bitnr, W. Durisch, G. Plfinger, F. von Roth, U. Vogt, A. Bronstrup, D. Seiler, Semiconductors, 38 (2004) 941.